1. Field of the Invention
The present invention is directed to three-dimensional surface profile imaging, and more particularly to a method and apparatus for three-dimensional imaging that uses color ranging to conduct surface profile measurement.
2. Description of the Related Art
A three dimensional surface profile imaging method and apparatus described in U.S. Pat. No. 5,675,407 (xe2x80x9cthe ""407 patentxe2x80x9d), the disclosure of which is incorporated herein by reference in its entirety, conducts imaging by projecting light through a linear variable wavelength filter (LVWF), thereby projecting light having a known, spatially distributed wavelength spectrum on the objects being imaged. The LVWF is a rectangular optical glass plate coated with a color-filtering film that gradually varies in color, (i.e., wavelength). If the color spectrum of a LVWF is within the visible light region, one edge of the filter rectangle may correspond to the shortest visible wavelength (i.e. blue or violet) while the opposite edge may correspond to the longest visible wavelength, (i.e. red). The wavelength of light passing through the coated color-filtering layer is linearly proportional to the distance between the position on the filter glass where the light passes and the blue or red edge. Consequently, the color of the light is directly related to the angle xcex8, shown in FIG. 1, at which the light leaves the rainbow projector and LVWF.
Referring to FIGS. 1 and 2 in more detail, the imaging method and apparatus is based on the triangulation principle and the relationship between a light projector 100 that projects through the LVWF 101, a camera 102, and the object or scene being imaged 104. As shown in FIG. 1, a triangle is uniquely defined by the angles theta (xcex8) and alpha (xcex1), and the length of the baseline (B). With known values for xcex8, xcex1, and B, the distance (i.e., the range R) between the camera 102 and a point Q on the object""s surface can be easily calculated. Because the baseline B is predetermined by the relative positions of the light projector 100 and the camera 102, and the value of xcex1 can be calculated from the camera""s geometry, the key to the triangulation method is to determine the projection angle, xcex8, from an image captured by the camera 102 and more particularly to determine all xcex8 angles corresponding to all the visible points on an object""s surface in order to obtain a full-frame 3D image in one snapshot.
FIG. 2 is a more detailed version of FIG. 1 and illustrates the manner in which all visible points on the object""s surface 104 is obtained via the triangulation method. As can be seen in the Figure, the light projector 100 generates a fan beam of light 200. The fan beam 200 is broad spectrum light (i.e., white light) which passes through the LVWF 101 to illuminate one or more three-dimensional objects 104 in the scene with a pattern of light rays possessing a rainbow-like spectrum distribution. The fan beam of light 200 is composed of multiple vertical planes of light 202, or xe2x80x9clight sheetsxe2x80x9d, each plane having a given projection angle and wavelength. Because of the fixed geometric relationship among the light source 100, the lens of the camera 102, and the LVWF 101, there exists a one-to-one correspondence between the projection angle (xcex8) of the vertical plane of light and the wavelength (xcex) of the light ray. Note that although the wavelength variations are shown in FIG. 2 to occur from side to side across the object 104 being imaged, it will be understood by those skilled in the art that the variations in wavelength could also be made from top to bottom across the object 104 or scene being imaged.
The light reflected from the object 104 surface is then detected by the camera 102. If a visible spectrum range LVWF (400-700 nm) is used, the color detected by the camera pixels is determined by the proportion of its primary color Red, Green, and Blue components (RGB). The color spectrum of each pixel has a one-to-one correspondence with the projection angle (xcex8) of the plane of light due to the fixed geometry of the camera 102 lens and the LVWF 101 characteristics. Therefore, the color of light received by the camera 102 can be used to determine the angle xcex8 at which that light left the light projector 100 through the LVWF 101.
As described above, the angle xcex1 is determined by the physical relationship between the camera 102 and the coordinates of each pixel on the camera""s imaging plane. The baseline B between the camera""s 102 focal point and the center of the cylindrical lens of the light projector 100 is fixed and known. Given the value for angles xcex1 and xcex8, together with the known baseline length B, all necessary information is provided to easily determine the full frame of three-dimensional range values (x,y,z) for any and every visible spot on the surface of the objects 104 seen by the camera 102.
As shown in FIG. 3, given the projection angle xcex8, the three-dimensional algorithm for determining the (x,y,z) coordinates of any surface spot Q(x,y,z) on a three-dimensional object is given below based on the following triangulation principle:                               x          =                                    B                                                f                  *                  ctg                  ⁢                                      xe2x80x83                                    ⁢                  θ                                -                u                                      *            u                          ,                  
                ⁢                  y          =                                    B                                                f                  *                  ctg                  ⁢                                      xe2x80x83                                    ⁢                  θ                                -                u                                      *            v                          ,                  
                ⁢                  z          =                                    B                                                f                  *                  ctg                  ⁢                                      xe2x80x83                                    ⁢                  θ                                -                u                                      *            f                                              (        1        )            
As a result, the three-dimensional imaging system described above can capture full-frame, high spatial resolution three-dimensional images using a standard camera, such as a charge coupled device camera, in real time without relying on any moving parts. Further, because the imaging system does not rely on a laser, it does not pose any hazard to the eyes when used in clinical applications. Also, because the wavelength of the light projected onto the object surface continuously varies, there is no theoretical limitation on the measurement accuracy that can be achieved by the system. The actual accuracy of a specific system will depend on system implementation and will be affected primarily by limiting factors such as the optical system design, the quality and resolution of the camera, light spectral emission of the light source projector; noise level and resolution of the frame grabber, calibration algorithms, and the three-dimensional imaging processing algorithms.
To avoid allowing the surface color of the object being imaged from affecting the imaging results, the system may obtain an image of the object under normal light conditions before projecting the filtered light onto the object. The image obtained under normal light conditions is then subtracted from the image obtained under LVWF light conditions to eliminate the effects of the object color on the image.
Even when the system compensates for the color of the object, however, the consistency of the spectral power distribution and the RGB value of each pixel may vary when light is projected onto the object through the LVWF based on the reflection characteristics of the object""s surface, particularly if the object is not white and/or not uniformly colored.
There is a need for a surface profile imaging method and apparatus that is able to generate consistent RGB values regardless of the reflection characteristics of the surface being imaged.
Accordingly, the present invention is directed to a method and apparatus for three-dimensional surface imaging that avoids variations in the RGB value of each pixel due to the reflection characteristics of the object""s surface. More particularly, a light source in the system illuminates an object or scene with a light pattern having a spatially varying wavelength and composed of at least one light plane. The light plane corresponds to at least one angle at which the light of that wavelength is emitted and contains only a single spectral component.
By imposing a single spectral light condition on the light source, the RGB values of each pixel will be independent of the light intensity of the light source and the reflectance characteristics of the object or scene being imaged. As a result, any color matching function that is conducted to link the wavelength of the light projected on the object or scene at a given point and that point""s position will be consistent, regardless of the color and/or reflectance characteristics of the object""s or scene""s surface.